I always get a perk from the plants with rain water, not with tap water. Tap water works, but rain water is better for the plants, and free too. Not a huge cost with tap water especially for only a few plants, but free is always better.
jacob
Yes, I noticed that as well. Notice my wording, that tap water is the "next best thing" and that it works "just about" as well. Not quite, but close.
The reason you mention is exactly why I worded it that way: my plants out in the garden seem to get growth spurts after a nice rain that they otherwise don't seem to get with plain tap water. But other than that, it doesn't seem to really make a difference.
I'm not sure if it's something to do with the water itself, or the fact that the plants get more thoroughly watered when it rains than they do when I do a quick watering with the watering can. I honestly have a feeling the duration of water (several minutes of evenly-spread rain vs. less than a minute drench with a watering can) has something to do with it, but some people in another thread on this forum brought up a good point: it could be that the rain is more acidic, unlocking extra nutrients to be used by the plants, or possibly the ionization of the water. But still, my local water is practically neutral in pH, so I wouldn't think it'd have that much of a difference, provided the pH of the soil is a bit lower. Another thing I'm thinking is, if it rains the sun's not out... therefore more of it is absorbed and used by the plants. If you water the plants on a hot and relatively sunny day, a lot of it evaporates. Even if the sun is not beating on the plants yet.
Ironically, all of my plants on the porch seemed to go through the same bursts of growth every couple days without ever getting any rainwater (and no fertilizer whatsoever until the beginning of July), while it seems that the plants in the garden seem to have their spurts only after a good, heavy rain. So I don't know... I think there may be a bit more to it. I also more thoroughly water the soil in the pots, simply because it's easier to do with just a watering can (no outside hose at this dump). But I watered the plants on the porch about every day or two, and the growth spurts were every 3-4 days... so really, I just think it's up to the plants. They seem to do whatever they wanna do.